The present invention relates to a control circuit that is applicable to power supply systems that use synchronous rectification techniques. The control circuit provides a self-driven method of control to an active switch by sensing the current flow over the switch. The active switch is a metal-oxide-semiconductor field-effect transistor (MOSFET). As disclosed, current is sensed first in the MOSFET's body diode for a very brief amount of time and then afterward the current is sensed in the MOSFET's on-state resistance. The switching action of the control circuit allows only one direction of current flow through the MOSFET, which, in turn, effectively synchronizes the active switch's turn-on with the conduction of the desired current direction.
Synchronous rectification is a method for improving efficiency of rectification by replacing a diode with an actively controlled switch, such as a MOSFET. This is especially applicable in high-current applications. A benefit from the use of a MOSFET as the switch is that the on-state resistance of a MOSFET has a very low resistance value that provides a low voltage drop when compared to a Schottky diode alone (which, e.g., typically has a 0.3 V or higher voltage drop). The lower voltage drop results in a lower power dissipation in each cycle, an increase in efficiency, as well as a reduction in size and cost due to the elimination of additional components including potentially bulky heat sinks required to cool the diode down.
However, in order to be able to use a MOSFET as a synchronous rectifier, voltage or current sensing is necessary. Because a MOSFET can conduct current in either direction, the timing of the turn-on and turn-off is significant in order to conduct current in the desired direction. Improper switching response may cause further losses that could limit or eliminate any benefit of replacing a diode rectifier with the MOSFET switch. Moreover, improper switching response may cause undesirable cross-conduction between a main switch and the synchronous rectifier resulting in potentially damaging shoot-through currents along a power supply. To maximize converter performance, the timing of the gate drive signal should be correct.
Some conventional synchronous rectifier controllers for driving synchronous rectifiers exist, however, such controllers typically are included within an integrated circuit with other features and functionality that limit the efficiency of the controller alone. Further, such controllers may be overly complex. As such, existing controllers are relatively expensive and not easily adaptable to a variation of uses. Beneficially, the control circuit disclosed herein includes a minimal number of components which makes the controller cost less to manufacture and more available. Further, by designing a simpler circuit for the purpose of driving a power MOSFET used as a synchronous rectifier at the correct timing by sensing the current flow at the MOSFET's on-state resistance, the disclosed controller is easier to implement in a wider array of uses than conventional designs.